Purpose: Hyponatremia is the most common electrolyte disorder and in 2006 the cost of treating hyponatremia in the US was estimated to be $1.6-$3.6 billion per year. Inappropriate vasopressin secretion is the major cause of dilutional hyponatremia associated with liver and heart failure. Brain derived neurotrophic factor (BDNF) and its receptor TrkB are expressed by magnocellular neurosectory cells that secrete vasopressin into circulation. Our studies will be among the first to test the role of the BDNF-TrkB signaling in an animal model of inappropriate vasopressin release. We propose that in an animal model of liver failure activity pendent stimulation of the BDNF-TrkB system increases vasopressin release by changing chloride transport preventing or reversing Cl- inhibition. Our hypotheses will be tested with the following Specific Aims: 1. To determine if norepinephrine inputs support chronic activation of AVP neurons in the SON of male bile duct ligated rats. 2. To determine if BDNF links the increased activation of AVP neurons to the changes in chloride transport loss of inhibition. 3. To test the role of estrogen in preventing AVP release in female BDL rats. Methods: The studies will employ Western blot and co-immunoprecipitation in combination with immunohistochemistry and laser capture microdissection RT-PCR, metabolism cage studies to measure urine and sodium excretion, DREADD, shRNA site-specific knockdown, and in vitro electrophysiology to test these hypotheses. Benefit: These experiments will address an existing gap in our understanding of neurophypophyseal function and the pathogenesis of hyponatremia. The findings of these experiments could potentially alter the way that inappropriate vasopressin release is studied and conceptualized clinically.